The present invention is concerned with semiconductor memory devices, which in particular relates to a phase-changeable memory device and method of programming the same.
The demand for semiconductor memory devices operable with random access in high integration density and large capacity is increasing. Flash memories, which are typically used in portable electronic devices, are typically regarded as such semiconductor memory devices. New kinds of proposed semiconductor memory devices each include a capacitor that is made of a nonvolatile material instead of the volatile material of a DRAM. Such devices include ferroelectric RAMs (FRAM) employing ferroelectric capacitors, magnetic RAMs (MRAM) employing tunneling magneto-resistive (TMR) films, and phase-changeable memories (or PRAM) using chalcogenide alloys. The phase-changeable memory devices, as nonvolatile memory devices, are able to be fabricated more readily than other memories and are advantageous in implementing large-capacity memories at low cost.
FIG. 1 is an equivalent circuit diagram illustrating a memory cell of a phase-changeable memory device. As shown in FIG. 1, a memory cell 10 of the phase-changeable memory device is composed of a variable resistor C and an access transistor M.
The variable resistor C is connected to a bitline BL. The access transistor M is connected between the variable resistor C and a ground voltage. A wordline WL is coupled to a gate of the access transistor M. When a predetermined voltage is applied to the wordline WL, the access transistor M is turned on to supply the variable resistor C with a current Ic through the bitline BL.
The variable resistor C contains a phase-changeable material (not shown). The phase-changeable material is conditioned in one of two stable states, i.e., a crystalline state or an amorphous state. The phase-changeable material changes into the crystalline state or the amorphous state depending on the current Ic that is supplied through the bitline BL. The phase-changeable memory programs data therein by means of such a characteristic of the phase-changeable material.
FIG. 2 is a graphic diagram showing a characteristic of a phase-changeable material. In FIG. 2, a reference number 1 denotes the conditioning curve to be the amorphous state while a reference number 2 denotes the conditioning curve to be the crystalline state.
Referring to FIG. 2, the phase-changeable material (GST) turns to the amorphous state when it is rapidly quenched after being heated at high temperature over its melting point Tm during a time Ti by current supply. The amorphous state is usually referred to as a reset state, storing data ‘1’. Otherwise, the phase-changeable material is settled in the crystalline state when it is slowly quenched after being heated at a temperature higher than crystallization temperature Tc and lower than melting point Tm during a time T2 longer than T1. The crystalline state is usually referred to as a set state, storing data ‘0’. The memory cell is conductive with resistance variable in accordance with the amorphous volume of the phase-changeable material. The resistance in the memory cell is highest in the amorphous state and lowest in the crystalline state.